While the core of naturally occurring ferritin is comprised of the hydrous iron oxide ferrihydrite, the ferrimagnetic core is typically comprised of the iron-oxide ferrite magnetite, Fe.sub.3 O.sub.4, or maghemite, .gamma.-Fe.sub.2 O.sub.3, or an intermediate composition. Magnetite and maghemite are members of a series continuum of solid solutions. For ease of description reference will be made to magnetite as inclusive of maghemite as the ferrimagnetic constituent of the ferritin core in the compositions of the invention. The core can also be comprised of other transition metal ferrites.
Ferritin, a protein naturally occurring in bacteria, fungi, plants, invertebrates and vertebrates including humans, is characterized by an ability to sequester and store iron in a bioavailable form. Chemically, the protein is a quaternary structure of 24 polypeptide units assembled into a spherical shell having an internal cavity about 8-10 nm in its longest dimension and penetrated by two types of intersubunit channels. In naturally occurring ferritin, the cavity contains up to 4500 iron atoms as a mineral core of the hydrous iron oxide ferrihydrite which is paramagnetic at ambient temperature. To my knowledge no one has substituted into the cavity of ferritin a ferrimagnetically ordered ferrite core with improved magnetic characteristics.